Chemical propellants are used extensively for rocket propulsion and gas generating systems. Traditionally solid and liquid chemical rocket propellants have been the primary means of powering rockets and missiles.
Typical rocket propellants are comprised of oxidiser(s) and fuel(s) in varying proportions to provide the required characteristics for a particular application.
In solid propellants, the fuel and oxidiser are stored in the same vessel with a portion of the fuel often acting as the means to bind its constituents together into a solid mass. This component is known as a binder fuel and typically comprises 10-20% of the mass. Binder-fuels traditionally used in solid propellant formulation are based on polybutadiene chemistry, with recent developments leading to binder-fuels based on glycidyl azide polymers. Other fuel binders include nitrocellulose plasticised with nitroglycerine, known as double based propellants.
Solid rocket propellants will often contain other additives including; thermic agents, high energy additives, bonding agents, curing catalysts or inhibitors, surfactants/emulsifiers and antioxidants. These additives are typically in the form of solids (usually fine powders), and/or non-volatile liquids which can be mixed together and then either (a) extruded or (b) undergo a chemical reaction to form the desired geometry of the final product (known as a grain.)
The chemistry of liquid propellants, by comparison is generally much simpler than solid propellants, often comprising of only one or two components. Hydrazine, liquid oxygen+liquid hydrogen (LOx-LH2) and liquid oxygen+kerosene (LOx/JP-1) are typical examples of this type of propellant. Liquid Propellants have their oxidiser(s) and fuel(s) stored in separate reservoirs which are intimately mixed with each other just prior to ignition. On a weight for weight basis, liquid fuels are significantly more powerful than their solid fuel counterparts.
Both traditional solid and liquid propellants have serious drawbacks which in some contexts prevent or seriously hamper their use.
With time, solid propellants will degrade via chemical and/or mechanical means which can lead to their failure during deployment, often with catastrophic consequences. Another drawback of solid rocket propellants is that the power output is predetermined by the geometry of the grain, and once ignited, they are not easily or reliably shut down or restarted if shutdown was successful. Solid propellants are often sensitive to heat, light, friction, and shock, and may ignite or explode unexpectedly if subjected to these conditions either at the time, or subsequently when attempted to be fired.
Liquid fuelled propellants suffer from three major drawbacks. Unlike solid propellants, at least one component of liquid propellants is a cryogenic liquid, requiring filling at a site shortly before use. In addition, expensive and heavy injection systems are required to homogeneously mix multiple liquid components just prior to combustion in a consistent, reproducible manner. Liquid propellants suffer from a very low density resulting in the need for large volume tanks being required to hold a relatively small mass of propellant.
As the name suggests, hybrid propellant systems were developed as a combination of the two systems. Like liquid fuelled rockets, hybrid systems have their fuel and oxidiser stored in separate compartments, however they exist in separate states of matter which are combined in the combustion chamber just prior to ignition. They are mechanically simpler and use denser fuels than liquid systems, and can be throttled, shut down and can potentially be restarted.
Even in view of the forgoing, hybrid propellant systems are far from ideal. The densities are still significantly less than solid propellants, and the system usually requires complex engineering to ensure the oxidiser:fuel ratio stays constant during propellant burn.
In summary, conventional solid & liquid propellant systems have considerable drawbacks. Solid propellants have little flexibility in operational output while liquid propellants have launch readiness and design drawbacks. Current alternative approaches, such as hybrid propellant systems, have not been able to completely remedy the problems associated with the earlier technologies.
It is an object of the present invention to provide an alternative form of propellant and, in particular, a viscous liquid monopropellant that overcomes at least some of the limitations of the abovementioned propellants, or which at least provides the public with a useful choice.